Production of refrigeration at very low temperatures has always presented a challenge. Heat must be removed at a very low temperature and converted so that it may be rejected at ambient temperature. Thermodynamically, the greater the difference between the rejection temperature and the final refrigeration temperature, the more energy will be required to remove a given quantity of heat. The challenges are to minimize the energy input to the refrigeration system and simplify the mechanical complexity of the system. The subject of this invention addresses both requirements.
Mechanical refrigeration systems typically circulate a fluid to transfer heat. The fluid is compressed to some pressure, which raises the temperature of the fluid enough so that heat may be rejected to some heat sink. The cooled, compressed fluid is then expanded to a lower pressure. If no energy is recovered in the expansion, then the expansion is adiabatic. This means that the enthalpy, the heat content, of the fluid is the same on both sides of the expansion device. Valves, orifices, and capillary tubing are typical devices used to adiabatically expand the fluid. If energy is recovered from the expansion, then the expansion approaches an isentropic process. This means that the entropy is nearly the same on both sides of the expansion device. Isentropic expansion is typically achieved by using a turbine or a reciprocating engine operated by the fluid. The expanded fluid drops in temperature due to the expansion and is able to absorb heat from a source at this lower temperature. The fluid warmed by the heat source may be further warmed by heat exchange with the pressurized fluid before being recompressed by the compressor.
Refrigeration systems which use a single component as the working fluid, in which condensation takes place at the high pressure, are limited by the physical properties of that component. For instance, with heat rejection taking place at ambient temperature, a minimum, practical refrigeration temperature is about -40.degree. C. Three methods are currently used to achieve lower temperatures: use of a non-condensing fluid, use of a fluid mixture, or a cascade refrigeration system. In each case the work required to achieve a liquid nitrogen temperature of 77.degree. K. is about ten times the heat energy removed.
The non-condensing fluid system is most efficient when the expansion is done isentropically. This means that the process is more practical in large scale systems.
In a fluid mixture refrigeration system, various compositions of the mixture are liquefied at the set high pressure and several temperature levels. At each temperature the liquid is separated from the fluid stream and flashed to a common low pressure. The flashed liquid is then heat exchanged with the gas stream to condense a new liquid stream at a lower temperature. The process is repeated until a flashed liquid is available at the desired refrigeration temperature. The type of components and amounts of each must be chosen so that a sufficient amount of liquid is formed at each of the temperature levels. To produce refrigeration at 77.degree. K., four liquid streams are typically produced and three to four liquid/vapor separators are required. Only one compressor is required to maintain the high and low pressures in the system.
A cascade utilizes multiple refrigeration systems operated in series. Each system uses a fluid component chosen for its performance over the operating temperature range of that system. The warmest system rejects heat at ambient temperature, and absorbs the reject heat from the next system in line. Each succeeding system operates over a colder temperature range until the desired refrigeration temperature is achieved.